Long DNA oligos containing the target sequence are cloned into LentiLox 3.7. Once clones have been isolated, virus is produced by transfecting 293 cells and collecting supernatant. This supernatant is then used to infect cells of interest directly, or concentrated for use in embryo infections. Note: These lentiviruses are pseudo-coated with VSV-G and are capable of infecting human cells, and thus present important biosafety issues.